All microscopes operate under a common set of principles, which can be described with reference to FIG. 3 of this application. To view the microscopic details of a specimen 305, it is placed on a specimen stage 310 in a microscope 300. An illumination light source 315 passes through a light condenser 320 before illuminating the sample 305. After passing through the sample, the light scattered by the sample is captured by the objective lens 325 and is passed to a scope or other imager for viewing. In addition to the magnification power of the objective 325, other factors can affect the quality of the magnified image. For example, the numerical aperture (NA) of the condenser 320 and objective 325 can greatly affect the resolution of the microscopic image of the specimen 305. The numerical aperture of a lens is defined by the equation NA=n*sin(.theta.) where n is the index of refraction of the lens and .theta. is the angular aperture of the lens, which is the angle between the centerline of the lens and a line from the focus point to the edge of a lens. To obtain the best resolution of an imaged specimen, the numerical aperture should be as high as possible and, importantly, the numerical aperture of the condenser (NA.sub.C) should be greater than or equal to the numerical aperture of the objective (NA.sub.O). A high numerical aperture means that light is directed to and collected from a wide variety of angles as it passes through the specimen. Since light is focused and collected from a variety of angles, the resolution of the microscopic image is greatly improved. Other factors that affect the quality of the imaged sample include the intensity of the illumination, the power of magnification, and the focal length of the lenses.
In recent years, interest has grown in using X-rays and other short-wavelength radiation as an illumination source for microscopy. X-ray microscopes use the same principles of microscopy that are described above, but instead use X-rays as an illumination source. X-rays have unique advantages over visible light and other wavelengths. X-ray wavelengths are much shorter than visible light wavelengths, thereby increasing the resolution of the microscope at high magnification. In addition, X-rays readily penetrate most materials or specimens, thereby improving the resolution of interior features of imaged specimens. Instead of using lenses that refract and focus light, X-ray microscopes use zone plate lenses to diffract light for focusing purposes. A representative example of a zone plate lens 400 suitable for this purpose is depicted in FIG. 4. The zone plate lens 400 depicted in FIG. 4 is a pattern of alternating opaque and transparent concentric regions. Each of the concentric regions has a smaller radial width as one moves towards the edge of the zone plate lens 400. This is because each region (opaque or transparent) in the zone plate lens 400 occupies the same area. The zone plate uses diffraction rather than refraction to focus the light that passes through it. In other words, the pattern of concentric rings creates a diffraction pattern that has its largest maximum at the first diffractive order (n=1). The zone plate also creates higher-order diffractive orders on each side of the first order (n=3, n=5, etc.). Each of these higher-order diffractive orders is less intense that the first order diffractive order by a factor of 1/n.sup.2. It is worth noting that when the light provided to a zone plate is perfectly collimated, the first order of diffraction will be found at the focal length of the zone plate, as shown in FIG. 4. Where the incoming light is not collimated, however, the first diffractive order will not be precisely aligned with the focal length of the zone plate.
One example of an X-ray microscope system 500 using these concepts is depicted in FIG. 5 and is described below. In FIG. 5, an X-ray source 505, such as a synchrotron, generates X-rays or other short-wavelength radiation. These X-rays pass through a long optical path so that the rays are nearly collimated by the time that they reach the condenser 515 of the microscope system 500. For example, where a radiation source 505 is placed 10-20 meters from the rest of the microscope system 500, the X-rays will only have a divergence of about 0.5 mrad. Reflecting devices, such as a plane mirror 510, can be used to extend the optical length of the X-ray source 505. The X-ray radiation is collected by a condenser zone plate 515, which creates a diffraction pattern with a maximum at its first order of diffraction. Since the incoming X-rays are nearly collimated at the condenser zone plate 515, the first order of diffraction will be nearly identical to the focal length of the condenser zone plate. For example, assuming a 20 meter distance from the X-ray source 505 and a 200 mm focal length for the condenser zone plate 515, the first diffraction order should be located at about 202 mm, which is close to the focal length of 200 mm. After passing through the condenser zone plate 515, the X-rays pass through a sample mounted on a sample stage 520 and are collected by an objective zone plate 525. The objective zone plate 525 also uses diffractive principles to focus the X-rays onto an imaging device, such as a CCD imager 530. Generally, the numerical aperture of the condenser zone plate 515 (NA.sub.C) should be greater than or equal to the numerical aperture of the objective zone plate 525 (NA.sub.O) in order to maximize the resolution of the microscope.
The X-ray microscope system 500 depicted in FIG. 5 includes several limitations. First, an X-ray source capable of generating sufficient power to be of interest for microscopy will generally require a synchrotron, which is a large, expensive, and cumbersome device to operate. Second, a long optical path is needed to ensure that that X-rays are nearly collimated when they reach the condenser zone plate. A long optical path adds significant size and heft to the device and also makes the device more susceptible to vibration and misalignment. Accordingly, a need exists for a more efficient and less bulky X-ray microscope system.